This invention relates generally to pellet burning systems and more particularly to burners suitable for use within a pellet burning heating appliance.
Pellets or fuel pellets, as those terms are used herein, are solid particulate fuels sufficiently uniform in size to permit convenient forcible feeding in bulk form, such as through a feed auger. Examples include pellets of compressed materials such as wood waste, nut shells, other celluosic products, small coal particles or the like which may be preformed into pellets, and naturally solid particles such as screened coal chips.
Typical pellet burning appliances are characterized by the burner system employed. Such systems often are described as overfed or underfed.
The overfed system is identified as such because fuel is dropped into the burner from above, usually six to eight inches. The burner, which may be shaped like a cup, receives the fuel wherein combustion takes place. Typical cup shaped burners have openings on the bottom and the sides, to introduce combustion air. This combustion air is also used to remove ash from the burner.
Most or all of the known overfed systems have several drawbacks. A major drawback is that the system can only burn clean fuel (fuel with a low ash content), because the combustion air can only remove light ash. If so-called "dirty" fuel, e.g., pellets of non-wood materials such as almond shells, pine bark, peanut shells, or other agricultural products with high silica content, is burned, clinkers (fused masses of non-combustible residues) are produced. The combustion air cannot remove these heavy clinkers. These clinkers build up and grow within the newly combusting fuel mass. The clinkers eventually fill or overlie the burner, whereby newly introduced fuel spills over the burner, and the fire is hindered or even goes out.
Another disadvantage of most overfed systems is that the burner must be cleaned often. This is a time consuming and dirty job, for clinkers must be removed and some residues must be scraped out of the burner.
In an underfed system, fuel is introduced into a burner below the zone of combustion. Typical burners in these systems are cup-shaped and have internal configurations to vertically deflect the fuel which typically is fed into the burner horizontally. Since the fuel moves vertically into the combustion zone, it pushes up most of the clinkers which then fall over the burner edge into an ash box or the like below. The larger clinkers which remain must be removed manually. This system is advantageous for it can burn dirty fuel and some of the clinkers will naturally fall over the burner edge into the ash box below.
A second advantage is efficiency. An underfed system is quite efficient since secondary combustion occurs in addition to primary combustion. Primary combustion occurs below the burner rim as fuel is fed into the combustion zone from below. This combustion is aided by primary combustion air, usually introduced half an inch to an inch below the rim of the burner.
Carbon dioxide is produced in the primary combustion zone. This carbon dioxide combines with glowing hot carbon (charcoal), produced from partially burned pellets, to produce two parts of carbon monoxide, a combustible gas. This carbon monoxide undergoes secondary combustion, which occurs at a point above the zone of primary combustion. Secondary combustion is aided by secondary combustion air, introduced immediately above the burner rim. Since primary and secondary combustion occur simultaneously, the flame temperature is higher, and burning is efficient.
Maintenance is a problem in many underfed systems. It must be done regularly, usually involving manually removing clinkers and scraping of residue formed out of some fuels. This is inconvenient or cumbersome in cup-shaped burners.